The present invention relates to the display of a digital audio signal embedded within a digital video signal, and more particularly to a digital audio waveform display of the embedded digital audio signal on a video waveform display instrument using circuitry already existing in the video waveform display instrument.
Digital audio signals may be embedded in digital video signals, as described in the proposed standard SMPTE-259M, by time multiplexing audio data words into available ancillary data space within the digital video signal data words. A video signal typically is digitized into ten-bit digital words. For composite video signals the video digital words representing a horizontal sync tip provide available space for inserting ancillary data. Typically four data words of a timing reference signal (TRS) and an identification number (ID) are inserted at the beginning of the horizontal sync tip in lieu of the video digital words, followed by 55 ancillary data words. The audio data words are inserted as the ancillary data words for each active horizontal line at the same time that TRS-ID is inserted. This provides a somewhat limited four channel capacity, which is a good match for current digital video tape recorders that record four channels of digital audio. Considerably more ancillary data space is available in component video, where only the active picture portion of the analog component video signal is digitized into digital video data words. The digital data words not carrying active video provide available space for inserting ancillary data, i.e., the space between the End of Active Video signal (EAV) and the Start of Active Video signal (SAV) between video lines of data. This provides room for 16 channels of audio data and more data of other types.
There is equipment available for displaying the video waveform content of the digital video signal, such as the 1730 D Series of Digital Waveform Monitors manufactured by Tektronix, Inc. of Wilsonville, Oreg., United States of America, but such equipment does not display the audio waveform content of any embedded audio signals. To provide a waveform display of the embedded audio signals would conventionally require separate equipment to extract and decode the digital audio signal with circuitry intended for decoding audio, and using this signal for display at audio rates with appropriate audio filtering.
What is needed is an inexpensive method for displaying the waveform content of digital audio data which is embedded in digital video signals using existing video waveform display circuitry.